With impetus growing for the long-awaited rail link, the Australian Pacific Airports Corporation has for the first time gone public with its preferred route, saying a line from Southern Cross to the airport with a major interchange at Sunshine is the “clear front-running route” for the project.

The company says it is enthusiastically advocating for a train line, which might cost up to $10 billion, as a much-needed alternative transport option for the 35 million air passengers who go through Tullamarine each year.

Prime Minister Malcolm Turnbull, who has pledged up to $5 billion in Commonwealth money to help build the line, and Premier Daniel Andrews met in Melbourne this month to discuss the project.

Mr Turnbull said he wanted to see the business case for airport rail produced by the state government by September so that key decisions can be made before November's state election.

With 120,000 vehicles heading for the airport each day, the recent Tullamarine Freeway upgrade has “bought us some time” in the battle to keep airport traffic moving, the airport’s chief of parking and ground access Lorie Argus told The Age.

But with passenger numbers expected to double to 70 million a year in the next 20 years, the transport picture will become “messy” if the rail link is not built, Ms Argus, the airport’s chief of parking and ground access, said.

The airport executive says the rail line has to be “frequent and affordable” if it is going to emulate other airport rail links around the world and carry millions of passengers each year.

There are three main options being considered for the rail’s alignment.

The Sunshine route, also preferred by the state government, would run fast trains from Southern Cross to Tullamarine through the western suburbs with the potential to link Geelong, Ballarat and even Bendigo into the network.

Another option is one of several variations of a route through Highpoint and Maribyrnong, creating a new suburban line and opening up new growth corridors in Melbourne’ booming north-west.

A third, less likely, suggestion is using the Craigieburn line with a spur between Essendon and Tullamarine.

Ms Argus said it was important that the business case should consider all options, but that she and her colleagues believed that west, through Sunshine, was best.

“For us, we’re looking at how to safeguard a rail to be able to extend beyond the airport or to be able to extend the network to the west,” she said.

“That’s why, for us, it looks like Sunshine would make sense to be the front running route because it has access to regional, it has access to the west where the major growth is.

“When you look at Skybus, that’s a pretty consistent 25-minute service, so at a minimum, we have a great mass-transit offer now.”

A continued failure to build the rail line would mean that roads to the airport would have to be continually upgraded and duplicated.

“The Tullamarine [Freeway] expansion buys us time, but if you project out our growth and you don’t have further road developments, that could look pretty messy in 2038,” Ms Argus said.